D. Tskhakaya

The plasma?sheath matching problem

The plasma–sheath matching problem has attracted new interest during the last few years. It is complicated by the singular structure of the asymptotic (λD/L → 0) plasma and sheath solutions and by a coupling with the eigenvalue problem originating from the plasma balance of bounded plasmas. Due to these difficulties the existence of a matched asymptotic expression uniformly valid from the plasma core to the wall is widely questioned. The issue is clarified both analytically and numerically by the explicit construction of a matched asymptotic expression and comparison with exact solutions for the hydrodynamic plane Tonks–Langmuir problem. The approximations obtained by consistent matching show excellent agreement with numerical potential curves. The singularities of the asymptotic components are reflected by small discontinuities in the derivatives that vanish in the limit λD/L → 0.

Theory of the plasma sheath in a magnetic field parallel to the wall

The plasma-wall transition (PWT) layer in a magnetic field parallel to the wall is investigated with a two-fluid approach, where the continuity and momentum equations for electrons and ions are used to describe the semi-bounded plasma, whereas the background gas of neutral particles is assumed to be uniformly distributed. The equations include ionization, recombination and charge exchange processes. The magnetic field is parallel to the confining wall, which is assumed to have a potential different from the one in the bulk plasma. A linear analysis of the PWT layer reveals two distinct length scales, namely the short Debye screening length scale and the much longer collisional length modified by the magnetic field. It is found that the ionization and recombination processes are important to connect the PWT layer to the bulk plasma, which is characterized by a balance between ionization and recombination. The PWT layer is here treated as a unit, without a priori splitting into sheath and presheath sublayer...

Optimization of PIC codes by improved memory management

A simple method is described for optimization of particle-in-cell codes by improved memory management. This includes a faster calculation of Monte-Carlo collision operators. It is demonstrated that the CPU time can be reduced by a factor of 2 and more without reduction of the simulation accuracy.

Link between fluid and kinetic parameters near the plasma boundary

In this paper the general problem of linking fluid and kinetic plasma parameters, with special attention devoted to the plasma boundaries where, due to strong deviations from thermodynamic equilibrium, there are intrinsic difficulties regarding the closure of the hydrodynamic equations, is considered. This problem is demonstrated by means of two examples for which the solutions of the kinetic equations are known. These examples are the collision-free Tonks-Langmuir model [Phys. Rev. 34, 876 (1929)] and Riemann’s presheath model [Phys. Fluids 24, 2163 (1981)] dominated by charge-exchange collisions. It is found that in the vicinity of the sheath edge the “polytropic” coefficient γ(x) shows an unexpected behavior that contradicts the commonly used hydrodynamic approaches assuming γ=const. In spite of all differences, the two models investigated exhibit quite similar behavior of the hydrodynamic quantities and of the polytropic coefficient in the presheath and sheath regions. This rises to hopes that the res...

ELM transport in the JET scrape-off layer

This contribution summarizes a number of aspects of the experimental and modelling programme at JET aimed at improving the characterization and understanding of edge localized mode (ELM) transport in the scrape-off layer (SOL). Divertor target energy deposition asymmetries favouring the inner target for the ion B × ∇B drift directed towards the X-point are observed with infra-red (IR) thermography. Similar trends are seen in the ELM resolved energy radiated in the divertor volume. Particle-in-cell kinetic calculations of the parallel ELM heat transport have been made for a range of ELM energies, revealing the detailed time response of target sheath heat transmission factors and indicating that electrons deposit ~30% of the ELM energy. The simulation results are in good agreement with experimental measurements of the integral energy deposited at the outer target up to the peak in target heat loads. A transient model of ELM filament energy evolution has been developed at JET and is able to reproduce a number of experimental observations, including the high ion energies observed in the far SOL using an electrostatic retarding field electrostatic analyser (RFA) and estimates of ELM heat fluxes deposited on main chamber limiters. During the ELM, the RFA and a second, SOL turbulence probe, clearly show the presence of coherent spikes in the hot ion flux, the plasma flux and the electron temperature. Field aligned structures have also been seen for the first time on JET in the power deposition on main wall limiters and upper dump plate surfaces using a new wide angle IR camera system. The probe signals are interpreted as the arrival of interspaced plasma filaments, with successive filaments carrying less energy. They are also consistent with the ELM out flux entering the SOL primarily on the outboard side and launching a sound wave disturbance along field lines.

Effects of energetic electrons on magnetized electrostatic plasma sheaths

The influence of energetic electrons on magnetized plasma sheaths is studied for different current regimes and different secondary-electron emission coefficients. (Here, the term “plasma sheath” denotes the collisionless region consisting of the non-neutral Debye sheath and the quasi-neutral magnetic presheath.) It is shown that the presence of even a small population of energetic electrons can significantly influence the potential drop across the sheath and the energy flux to the wall. For example, for plasma parameters typical of contemporary tokamaks, the presence of a fast-electron population with density smaller than 0.1% (!) can double the potential drop across the sheath and the energy flux to the wall, and the presence of a few percent of fast electrons can enhance these values by up to one order. The effect of fast electrons decreases with increasing secondary-electron emission coefficient and increasing current to the wall. Analytical results obtained are checked against particle-in-cell (PIC) s...

Radio-frequency discharges in oxygen: I. Particle-based modelling

In this series of three papers we present results from a combined experimental and theoretical, particle-based study to quantitatively describe capacitively coupled radio-frequency discharges in oxygen. The particle-in-cell Monte Carlo model on which the theoretical description is based is described in this paper. It treats space charge fields and transport processes on an equal footing with the most important plasma–chemical reactions. For given external voltage and pressure, the model determines the electric potential within the discharge and the distribution functions for electrons, negatively charged atomic oxygen and positively charged molecular oxygen. Previously used scattering and reaction cross section data are critically assessed and in some cases modified. To validate our model, we compare the densities in the bulk of the discharge with experimental data and find good agreement, indicating that essential aspects of an oxygen discharge are captured.

JET disruption studies in support of ITER

Plasma disruptions affect plasma-facing and structural components of tokamaks due to electromechanical forces, thermal loads and generation of high energy runaway electrons (REs). Asymmetries in poloidal halo and toroidal plasma current can now be routinely measured in four positions 90° apart. Their assessment is used to validate the design of the ITER vessel support system and its in-vessel components. The challenge of disruption thermal loads comes from both the short duration over which a large energy has to be lost and the potential for asymmetries. The focus of this paper will be on localized heat loads. Resonant magnetic perturbations failed to reduce the generation of REs in JET. An explanation of the limitations applying to these attempts is offered together with a minimum guideline. The REs generated by a moderate, but fast, Ar injection in limiter plasmas show evidence of milder and more efficient losses due to the high Ar background density.

Effect of E × B Drift on the Plasma Flow at the Magnetic Presheath Entrance

The boundary condition for ion fluid velocity at the magnetic presheath entrance is derived in the presence of the E x B drift. This condition takes into account gradients of the drift velocity and differ from the classical Bohm-Chodura condition.

Particle-in-cell simulations of the plasma-wall transition with a magnetic field almost parallel to the wall

Abstract Using simple analyses and 1d3v (one space and three velocity dimension) particle-in-cell simulations we show that the properties of the plasma-wall transition change when the angle α between the wall and the magnetic field is of the order of, or smaller than, certain values α1, α2. These values depend on the ion mass and collisionality, and on the ion-to-electron temperature ratio in the magnetic presheath. In this range of α, no significant decrease in the particle and heat fluxes to the wall is expected with decreasing α.